Zeus is very difficult to detect even with up-to-date antivirus software due to being Stealthy. This is the primary reason why its malware family is considered the largest botnet on the internet: Some 3.6 million PCs are said to be infected in the U.S. alone. New River Computing advises businesses to train users not to click hostile or suspicious links in emails or on the web while also keeping up with antivirus and software updates.